Cooling acetylated wood elements

ABSTRACT

A process for the production of acetylated wood elements, a cooling system and a wood acetylation plant are described. A process for the production of acetylated wood elements comprises acetylating wood elements and cooling the acetylated wood elements wherein the cooling comprises supplying liquid water to the acetylated wood elements to provide wetted wood elements and exposing the wetted wood elements to a gas flow.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 16/312,525,filed Dec. 21, 2018, which is a U.S. National Stage of InternationalApplication No. PCT/EP2017/065520, filed Jun. 23, 2017, said patentapplications being fully incorporate herein by reference.

FIELD OF THE INVENTION

The invention pertains to a process for the production of acetylatedwood elements comprising a cooling step. In the cooling step, woodelements are cooled from a temperature preferably in the range of 120 to180° C. to preferably less than 60° C.

BACKGROUND OF THE INVENTION

Acetylation of wood has long been recognized as a method to improve thedurability of otherwise non-durable softwood species. Acetylation can beused to improve wood properties, such as hardness and dimensionalstability. Background references for such processes include WO2013/117641 and WO 2013/139937. Acetylating wood generally involvescontacting wood elements with acetylation fluid comprising aceticanhydride and/or acetic acid while heating. The acetylation fluid isgenerally recirculated.

Acetylation of wood provides hot acetylated wood elements with atemperature of, typically, at least 120° C. or at least 150° C., oftenbetween 160 and 180° C. The acetylated wood elements are hence too hotfor storage and further handling. Storage of acetylated wood elementswith such temperatures involves a risk of smoldering (flamelesscombustion), even while the wood elements are slowly cooling againstambient. Cooling of the acetylated wood elements is therefore necessary,in particular if the acetylated wood elements are not immediatelyfurther processed.

However, current cooling methods are unsatisfactory for (acetylated)wood elements. The low heat transfer coefficient of wood elements makescooling challenging. In particular for gas cooling and cooling withindirect heat exchange, the low heat transfer coefficient causesproblems. Cooling by flowing cooling gas over the acetylated woodelements is not efficient enough and would require very high gas flowsand long residence times of the wood elements. Indirect cooling does notprovide for homogenous and fast cooling in view of the very poor heattransfer from cooled equipment to wood elements. A further method ofsubmerging acetylated wood elements in a cooling bath is not attractivebecause this would result in wet acetylated wood elements. The cooledwood elements may even be dripping. Moreover, there is a risk ofleaching of residual acetylation fluid from the acetylated wood elementsinto the cooling liquid.

Accordingly, there is a need for better cooling methods for inparticular acetylated wood elements and processes incorporating suchmethods. Desirably, these methods are fast and efficient and do notinvolve the above-mentioned disadvantages of prior art methods.

JP H06-198610 describes a process wherein wooden fiber obtained byshredding wood is acetylated in a liquid phase, then this acetylatedwooden fiber is accumulated and formed in one piece.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention provides, in an aspect, a process for the production ofacetylated wood elements, comprising acetylating wood elements andcooling the acetylated wood elements wherein the cooling comprisessupplying liquid water to the acetylated wood elements to provide wettedwood elements and exposing the wetted wood elements to a gas flow.

In a further aspect, the invention relates to a cooling systemcomprising a water spray chamber, an evaporation section downstream ofsaid water spray chamber, and a gas recirculation loop, wherein saidwater spray chamber comprises an inlet for wood elements, a liquiddistributor for spraying water, and an outlet for wood elements, andwherein the evaporation section comprises an inlet and an outlet forwood elements, a conveyor for continuously transporting wood elementsfrom said inlet to said outlet, and an inlet conduit having at least oneopening for introducing gas into said evaporation section and an outletconduit having at least one opening for withdrawing gas from into saidevaporation section, wherein said conduits are coupled to said gasrecirculation loop and wherein said openings of said conduits aredifferent from said inlet and outlet for wood elements.

The invention also relates to a wood acetylation plant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process scheme for a non-limiting illustrative embodimentof a process for the cooling of wood elements according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention generally relates to the production of acetylated woodelements and to the cooling of acetylated wood elements. The inventionis in an aspect based on the judicious insight that a combination ofwetting of hot acetylated wood elements with a liquid and evaporatingthe liquid from the wetted wood elements provides efficient cooling ofacetylated wood elements. Accordingly, the process comprises supplying aliquid, preferably water, to the acetylated wood elements to providewetted wood elements and exposing the wetted wood elements to a gasflow. The exposure to the gas flow causes evaporation of the aqueousliquid which is in contact with the wood elements, resulting in atemperature decrease of the wood elements. In the present invention, thewetting and exposing are used to provide for evaporative cooling of hotacetylated wood elements. This may be contrasted to for example airconditioning methods using evaporative cooling to cool air.

The process comprises acetylating wood elements. The step of acetylatingwood elements generally involves contacting wood elements withacetylation fluid comprising acetic anhydride and/or acetic acid. Duringthe contacting, the temperature of the wood elements increases to atemperature in the range of 120 to 180° C., more preferably to a finaltemperature in the range of 150 to 180° C.

The acetylation step is preferably a continuous process step. Theprocess can be a batch or continuous process or comprise a combinationof batch and continuous process steps. Preferably, the cooling step is acontinuous process step. More preferably, the exposure step is acontinuous process step.

Preferably, the acetylation comprises removal of excess acetylationliquid from the acetylated wood elements, for example under reducedpressure. Preferably, the process comprises a finishing step between theacetylation and the cooling step. The finishing step generally has thepurpose of reducing the content of non-reacted acetic anhydride andformed acetic acid of the wood elements. A finishing step preferablycomprises exposing acetylated wood elements to reduced pressure and/orhigh temperatures, such as about 130° C. or higher. Frequently, the woodelements as obtained from a finishing step have a temperature of morethan 120° C., such as more than 150° C.

Preferably, the acetylated and optionally finished wood elements have atemperature of 120° C. or more, such as 150° C. or more, and preferablyan acetyl content of more than 15 wt. % or more than 17 wt. % or morethan 20% or more than 21 wt. %; preferably at its geometrical centre.Preferably the acetylated wood elements also have a residual acidcontent of less than 1 wt. %, or less than 0.9 wt. % or less than 0.5wt. %, on dry basis, for example obtained by a finishing step.Preferably, the cooled wood elements have such acetyl content and/orresidual acid content.

The acetyl content of the wood can be determined as follows. Samples areground to wood particles. From these samples residual trace amounts ofacetic acid and/or acetic anhydride are removed by washing with waterand subsequent drying at 103±2° C. for between 14 to 24 hrs. Afterweighing these dried samples, the acetyl groups are released from thewood in the form of acetate ions by saponification with sodium hydroxidesolution at elevated temperature, typically 90° C. This saponificationreaction runs for 4 hrs, with stirring every 15 minutes. The acetateions are quantified by means of high-pressure liquid chromatography(HPLC), after calibrating this HPLC with standard acetate solutions andusing sodium butyrate as internal reference.

The residual acetic acid (RA) content is a measure of the residual,non-bound acetic acid contained in the wood (including acetic acidformed by hydrolysis of residual, non-bound acetic anhydride). Aceticacid may also originate from the wood itself, therefore the RA measuresboth the original acetic acid and the acetic acid left from theacetylation reaction. For determining the residual acid content (RA) awell-defined amount of 3-5 g of sample material is shaken indemineralised water for 1 hr. After this extraction step the sample isseparated from the water fraction by filtration. Subsequently this waterfraction is titrated with a known sodium hydroxide (NaOH) solution,using phenolphthaleine as an indicator, from which the residual acidconcentration of the sample can be calculated.

The process comprises cooling the acetylated wood elements. Preferably,the wood elements are cooled to a temperature below 100° C. or below 80°C., more preferably below 60° C. or below 40° C., such as to ambienttemperature, generally above 5° C. Preferably, the temperature of thewood elements decreases by at least 30° C. or at least 50° C., morepreferably by at least 80° C. during said cooling, more in particularduring the exposure to the gas flow. Preferably, the wood elements atthe outlet of an evaporation section have a temperature below 100° C. orbelow 80° C., more preferably below 60° C. or below 40° C., for exampleambient temperature. In case an atmosphere different from ambient ismaintained in an evaporation section, e.g. an inert atmosphere, then anoutlet of an evaporation section is optionally characterized by transferof wood elements to ambient atmosphere, and/or to an atmospheredifferent from the gas flow. Preferably, this cooling is achieved inless than 30 minutes or less than 15 minutes.

The cooled wood elements can for instance have a water content of lessthan 35 wt. %, for example in the range of 1 to 8 wt. % or 5 to 10 wt.%, in particular at the outlet of an evaporation section.

The invention generally relates to producing acetylated wood elements.In particular wood chips, strands and particles are challenging to coolcompared to e.g. wood fibres in view of the lower specific surface area(m² surface area/kg wood).

The wood elements for example comprise or consist of one or moreselected from the group consisting of wood chips, wood strands, and woodparticles. The process and system of the invention are for example usedfor the acetylation of durable and non-durable hardwoods, as well asdurable and nondurable softwoods. The wood elements preferably belong tonon-durable wood species such as non-durable hardwoods, or soft woods,for example, coniferous trees, typically spruce, pine or fir. Preferredtypes of wood are spruce, sitka spruce, maritime pine, scots pine,radiata pine, eucalyptus, red alder, European alder, beech and birch.

Typical dimensions of some types of wood elements are given in thefollowing table 1. Preferred are wood chips, strands, and particles asdefined in table 1 and optionally slivers. Preferably, the wood elementsconsist of one of the types of wood elements as defined in table 1, toachieve more homogenous cooling. In some embodiments, wood particleshave a width and/or thickness of 1.0 to 5.0 mm. Preferably, the woodelements have at least two dimensions (in orthogonal directions) of atleast 0.15 mm.

TABLE 1 Typical dimensions of types of wood elements Type of length (mm)width (mm) thickness (mm) wood element from to from to from to Chips 575 5 50 1.5 25 Strands 20 120 5 40 0.25 1.5 Splinters (slivers) 5 750.15 0.5 0.15 0.5 Particles 1.5 20 0.15 5 0.15 5 Fibre bundles 1.5 250.15 0.5 0.15 0.5 Fibres 1 5 0.05 0.1 0.05 0.1

The cooling comprises supplying liquid water to the wood elements toprovide wetted wood elements. The water is optionally supplied as liquidstream comprising other components, or as liquid stream essentiallyconsisting of water, such as for more than 99 wt. %.

Preferably, liquid water is provided on the surface of the wood elementsand preferably the wood elements are brought in contact with liquidwater. Preferably, the process comprises introducing wood elements intoa wetting chamber and introducing liquid water into the wetting chamber.In some embodiments, the wetting does not involve condensation in thewetting chamber. In some optional embodiments, the process for exampledoes not comprise introducing steam into the wetting chamber in contactwith wood elements. Preferably, the amount of liquid water supplied tothe wood elements is in the range of 10 to 500 g water, more preferably50 to 400 g water, or 100-500 g water, even more preferably 100 to 250 gwater, such as 100 to 150 g water per 1 kg dry wood elements.Preferably, the wood elements are contacted with 10 to 500 g water per 1kg dry wood elements, more preferably 50 to 400 g water, even morepreferably 100 to 250 g water, such as 100 to 150 g water per 1 kg drywood elements. This provides for better cooling. Preferably, a streamcomprising wood elements at an outlet of the wetting chamber comprisessuch amounts of water per kg dry wood, and still comprises such amountsat the inlet of the evaporation section. Preferably, the gas flow iscontacted with a stream comprising wood elements having such amounts ofwater.

The temperature of the liquid water as introduced into the wettingchamber is for example in the range of 5 to 95° C., such as between 10and 50° C. The wetting is preferably performed at ambient pressure suchas between 1 and 5 bara. Preferably, the wetted acetylated wood elementshave a temperature of 100° C. or more, or 120° C. or more, such as 150°C. or more.

Preferably, wetted acetylated wood elements are maintained under lowoxygen conditions, such as under inert gas and/or vacuum, from theacetylation step to inlet of the wetting chamber. Herein, low oxygenconditions include an oxygen partial pressure of for example less than 5kPa or less than 2 kPa. This advantageously allows for preventingcharring of the wood elements. Accordingly, the wetted acetylated woodelements are hot, e.g. 120° C. or more, such as 150° C. or more, fromthe acetylation step to inlet of the wetting chamber and also at theinlet of the wetting chamber.

The hot acetylated wood elements may be transported through the wettingchamber for instance by gravity flow or using a conveyor.

Preferably, a part or all of the water is supplied to the wood elementsby spraying, preferably to provide homogenously wetted wood elements.Spraying also provides for limited sensible heat exchange during wettingof wood elements. Preferably, the wood elements are wetted in a chambercomprising at least one liquid distributor for spraying water.Preferably, the water is sprayed through nozzles. Optionally, water issprayed as droplets having a volume mean diameter of at least 100 μm, orat least 250 μm, such as in the range of 250 to 550 μm. In the preferredprocess, spraying is used for distributing and providing liquid waterover the surfaces of the hot acetylated wood elements and in the voidsbetween hot acetylated wood elements, rather than to promote evaporationof droplets. Spraying may include showering water, for example with amoving head. Optionally, less than 10 wt. % of the introduced liquidwater evaporates in the wetting chamber, optionally without contact withacetylated wood elements.

The cooling further comprises an evaporation step comprising exposingthe wetted wood elements to a gaseous medium, in particular to a gasflow, usually in an evaporation section. Preferably, the acetylatedwetted wood elements at the beginning of such exposure to the gas flowand/or at the inlet of such an evaporation section have a temperature of120° C. or more, such as 150° C. or more As a result of this exposure,water evaporates from the wood elements, and the required heat ofvaporisation is withdrawn from the wood elements, thereby decreasing thetemperature of the wood elements. Preferably, during the exposure to thegas flow, the temperature decreases by at least

Preferably, the wetted wood elements are exposed to a gas flow such thatwater vapour is continuously removed from the wood elements. Preferably,the wood elements are exposed to a gaseous flow having a relativehumidity of less than 50% or less than 20% or less than 10% at thetemperature of the wetted wood elements. More in particular, this refersto water content of the gas flow at an inlet of the evaporation section,calculated as relative humidity based on the temperature of the woodelements at the inlet for wood elements of the evaporation section. Thegas flow at the inlet generally has a temperature at least 10° C. or atleast 20° C. or at least 50° C. or at least 100° C. lower than the woodelements introduced into the evaporation section.

The evaporation section is preferably operated at a pressure in therange of 0.5 to 5.0 bara, such as between 1 and 2 bara, preferably atambient pressure.

Preferably, the wetted wood elements are conveyed through an evaporationsection, preferably in a continuous manner. The wetted wood elements mayalso be exposed to an air flow while not moving. The residence time ofthe wood elements in the evaporation section is for example less than 30minutes or less than 15 minutes or less than 10 minutes, such as in therange of 5 to 10 minutes.

Preferably, the gas flow has a lower oxygen concentration than ambientatmosphere, so as to not oxidise the hot wetted acetylated woodelements. The hot wetted acetylated wood elements are e.g. 120° C. ormore, or 150° C. or more. Hence, generally, the gas flow (irrespectiveof oxygen content) is contacted with hot wetted acetylated wood elementsof e.g. 120° C. or more, or 150° C. or more. Preferably, the gas flowcomprises, at the inlet into the evaporation section, less than 10%oxygen by volume, or less than 5% or less than 2% or less than 1% oxygenby volume. More preferably, the gas flow comprises at least 90% or atleast 99% by volume of inert gas such as nitrogen, carbon dioxide and/orflue gas. Nitrogen is preferred. Also possible is using oxygen depletedand/or nitrogen enriched air, compared to ambient. In view of thesepreferred compositions of the gas flow, the gas is preferablyrecirculated.

Preferably, the evaporation step comprises providing a gas flow over thewood elements, preferably of inert gas, in an evaporation sectionwherein the wood elements are conveyed in a continuous manner. Theconveying direction defines the length of the evaporation section.Hence, a gas flow having a flow direction is introduced in theevaporation section, in particular into the space through which the woodelements are conveyed. The gas flow is further vented from theevaporation section. The gas flow generally has a direction essentiallyco-current, counter-current or perpendicular (cross-current flow) to theconveying direction of the wood elements. Cross-current flow ispreferred. The gas flow direction is defined by the one or more inletsand one or more outlets for gas of the evaporation section, which may beseparate from the inlet and outlet for wood elements.

In some embodiments, the wood elements are horizontally conveyed in theevaporation section. The gas flow is for example horizontally from sideto side, or from top to bottom, or from bottom to top of (and inside)the evaporation section. Preferably, said evaporation section comprisesa conveyor system comprising the inlet and outlet for wood elements, aconveyor and a shell, wherein the shell provides a space through whichwood elements are conveyed by the conveyor (such as a belt, chain orscrew). The shell is configured for maintaining an atmosphere in thespace different from ambient. Preferably, the evaporation sectioncomprises inlets for the gas flow into the space and outlets for ventingor withdrawing gas comprising water vapour from the space. Preferably,the openings of these inlets and outlets for gas flow are different insize to the inlet and outlet for wood elements. Preferably, they arepositioned between the inlet and outlet for wood elements. Optionally,these openings are provided through the shell. Optionally, theseopenings are provided in gas conduits provided in the evaporationsection and generally extending in said space and connected to anopening in said shell, or extending to or through an inlet or outlet forwood elements. Preferably, the openings for gas have a smaller size thanthe inlet and outlet for wood elements, and preferably have a sizesmaller than the size of the wood elements. Preferably, the inlet gasconduit comprises a manifold for distributing the gas flow over at leastpart of the length of the evaporation section. Preferably the openingsand the inlet and outlet for wood elements are arranged for cross-flow.

A screw conveyor is preferred, as this may provide for homogenisation ofthe wood elements, in addition to transport. The screw action mayprovide for increased homogeneity of the temperature of the woodelements. This may advantageously allow for a reduced residence time.Moreover, the screwing movement may allow for mixing of wood elementsand water, such as by shuffling and/or tumbling. The screw movement mayalso allow for redistribution of water over the wood elements, inparticular from slowly drying wood elements to quickly drying woodelements. The screwing movement may also provide for optimising exposureof the wood element surfaces to the drying atmosphere. A horizontalscrew conveyor is preferred. A filling level of for example at least 70%or at least 80% volumetric fill can be used to give homogeneous flow andprevent shortcutting of gas flow through low fill sections. Beltconveyors, tubular belt conveyors, bucket conveyors, and chain conveyorscan also be used.

The process preferably comprises withdrawing gas comprising inert gas,preferably saturated or nearly saturated with water vapour, from anevaporation section. The gas is preferably recirculated using arecirculation loop coupled to the evaporation section. In therecirculation loop, the gas is preferably cooled, water vapour iscondensed from the gas, the condensate is preferably removed from therecirculating gas, and the gas is recirculated back to the section usinge.g. a fan. A purge of gas may be withdrawn and/or gas may be added inorder to control pressure and oxygen level. By condensation of watervapour in the recirculation loop and removal of condensate, the relativehumidity and temperature of the recirculated gas stream are at leastpartly controlled and the drying capacity of the gas flow is maintained.The amount of condensate also indicates the effective withdrawal of thelatent heat of vaporisation from the wood elements. Accordingly,preferably, at least 3 wt. % or at least 5 wt. % water is condensed inthe recirculation loop based on weight of the wood elements, e.g. perminute, based on kg condensate formed and preferably withdrawn per kgwood on dry basis, e.g. per kg wood elements passing through the outletof the evaporation section.

Optionally, the gas flow is passed through the evaporation sectionmultiple times before being subjected to condensation, for examplethrough separate zones arranged in series in the evaporation section.Optionally, cross-current flow within a zone is combined with overallcounter-current flow through multiple zones.

The cooling in the recirculation loop preferably comprises indirect heatexchange against a cooling fluid. Optionally, the heat of condensationreleased during the condensation is recovered e.g. by heat exchangeagainst a fluid stream to be heated. Furthermore, the condensate isseparated from the gas and is optionally recycled to the wetting step,or for example further processed and disposed of. The recirculation looppreferably comprises a fan for reintroducing the dried gas as gas flowinto the evaporation section. Preferably, the temperature decrease ofthe gas in the recirculation loop is at least 5° C. or at least 10° C.Preferably, the water vapour content of the reintroduced gas flow isless than 50% of the water vapour content as the gas flow withdrawn fromthe evaporation section.

Optionally, the process further comprises storing and/or handling thecooled wood elements. The described cooling may also be used for coolingwood elements that are not acetylated.

Yet a further aspect of the invention relates to a cooling system (1). Anon-limiting embodiment is shown in FIG. 1 . The cooling system (1)comprises a water spray chamber (2), an evaporation section (6)downstream, with respect to wood elements, of said water spray chamber(2), and a gas recirculation loop (11). The water spray chamber (2)comprises an inlet (3) and an outlet (5) for wood elements (3), andpreferably a liquid distributor (4) for spraying water. The downstreamevaporation section (6) comprises an inlet (7) and an outlet (13) forwood elements, a shell (9), and preferably a conveyor (8) forcontinuously transporting wood elements through said shell (9) from saidinlet (7) to said outlet (13). Said outlet (5) is connected to saidinlet (7). Preferably, the evaporation section (6) is provided withopenings (10) for introducing gas into evaporation section (6) andwithdrawing gas from evaporation section (6). These openings are coupledto said gas recirculation loop (11), e.g. in fluid communication. Theopenings (10) are different and spaced apart from the inlet (7) andoutlet (13) for wood elements. Preferably, the evaporation sectioncomprises an inlet conduit (10 a) and an outlet conduit (10 b) for gascomprising such openings (10). The conduits are coupled to the gasrecirculation loop (11). The conduits (10 a, 10 b) are providedpreferably in said shell, e.g. at least partly through said shell or atthe inside wall of said shell. The inlet conduit (10 a) allows fordistributing gas over and into at least part of the space defined by theshell (9). The outlet conduit (10 b) allows for collecting gas, e.g.over the length of the shell, and withdrawing gas from the shell (9)into the recirculation loop (11). Both conduits preferably have a walldistinct from the shell, wherein said openings (10) are provided in saidwall.

Preferably, the evaporation section (6) is constructed in a screwconveyor, wherein the conveyor (8) is at least one screw. Preferablyopenings (10) are arranged in said screw conveyor for flow of gas incross-flow with the conveying direction of said screw.

Preferably, said recirculation loop (11) comprises a cooler (12) and acondensation vessel (14), which may also be combined with each other,and has an outlet for condensate (15). Preferably the recirculation loopfurther comprises a recirculation fan (16) for reintroducing the gasflow into the evaporation section. The recirculation loop may furthercomprise an inlet for gas (17) and optionally a gas purge outlet (notshown). The gas recirculation loop is coupled in fluid communicationwith said evaporation section, in particular with the space inside saidshell through which wood elements are conveyed.

Preferably, the gas inlet conduit (10 a) is arranged radially opposed tothe gas outlet conduit (10 b). Herein, the radial direction isperpendicular to the length of the evaporation section. Optionally, agas conduit extends parallel to the conveying direction over at leastpart, preferably more than half, of the length of the evaporationsection, wherein the gas conduit is provided with openings, preferablydistributed over at least part of its length and more preferably over atleast half of the length of the evaporation section.

For example a manifold can be used as a gas inlet conduit. Thisadvantageously provides for introducing the gas with a good distributionover the conveying direction. Optionally, a wedge-wire screen is used asa gas outlet conduit.

The shell (9) and/or outlet (13) are preferably configured formaintaining an atmosphere different from ambient atmosphere in theevaporation section, in particular for maintaining a lower oxygenpartial pressure. Further preferences for the cooling system (1) are asdescribed in connection with the process. In operation, wood elementsare acetylated in section (18) and wetted by spraying water on it inchamber (2). The wetted wood elements are exposed to a gas flow beingwhile conveyed through the evaporation section (6) such that waterevaporates. Water vapour condenses in condensation vessel (14). Cooledwood elements such as wood chips are obtained at the outlet (13).

An evaporation section optionally comprises one or more evaporationunits, or optionally comprises one or more zones provided in a unit orshell which also comprises other zones, such as a wetting zone. Theevaporation section optionally comprises one or more evaporation zoneswhich for example have different temperature, pressure, and/orcomposition of gas flow. The cooling system can be used in a method forproducing acetylated wood elements as described. The cooling system canbe used in a method of cooling wood elements that are acetylated or arenot acetylated.

In yet a further aspect, the invention also relates to a woodacetylation plant comprising a wood acetylation section (18) anddownstream thereof said cooling system (1), wherein the outlet of saidwood acetylation section is coupled for transport of wood elements tosaid inlet (3) for wood elements of said water spray chamber.Preferably, the wood acetylation plant is configured for transport ofwood elements from said acetylation section to said cooling systemwithout exposure of the wood elements to ambient air and/or undercontrolled atmosphere.

The wood acetylation section comprises a reaction unit, such as achamber or vessel or conveying unit. Preferably, said wood acetylationsection further comprises a heater for heating acetylation fluid. Thewood acetylation section preferably further comprises a part downstreamof said reaction chamber configured for removing acetylation fluid.Preferably, said reaction chamber is designed so as to provide forcontinuous transport of wood elements from the inlet to an outletthereof. Optionally, said reaction unit comprises a vertically arrangedplug flow reactor. Optionally, said reaction unit comprises a screwconveyor, preferably substantially horizontally arranged (includingoblique), for contacting wood elements with acetylation fluid. The plantmay comprise an impregnation unit upstream of the acetylation sectionconfigured for vacuum impregnation of wood elements with acetylationfluid.

For the purpose of clarity and a concise description features aredescribed herein as part of the same or separate embodiments, however,it will be appreciated that the scope of the invention may includeembodiments having combinations of all or some of the featuresdescribed. It will be clear to the skilled person that the invention isnot limited to any embodiment herein described and that modificationsare possible. The preferred and exemplified features of the processesand the apparatus and system can be combined with each other. Theprocess of the invention optionally comprises a wood acetylation processor method. The term “comprising” is used to allow for the presence offurther elements and/or steps than those recited; and encompassespreferred embodiments wherein the referenced item substantially oressentially consists, or consist, of the recited elements and/or steps.Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to‘only one’, but instead are used to mean ‘at least one’, and do notexclude a plurality. Features that are not specifically or explicitlydescribed or claimed may be additionally included in the structure ofthe invention without departing from its scope. The use of expressionslike “preferably” is not intended to limit the invention.

1. A process for the production of acetylated wood elements, comprising:acetylating wood elements; and cooling the acetylated wood elements, thecooling comprising: supplying liquid water to the acetylated woodelements to provide wetted wood elements; and exposing the wetted woodelements to a gas flow.
 2. The process according to claim 1, wherein thegas flow is contacted with a stream of the wetted wood elementscontaining 10 to 500 g water per 1 kg dry wood elements.
 3. The processaccording to claim 1, wherein the wood elements have at least twodimensions of at least 0.15 mm.
 4. The process according to claim 1,wherein the wood elements comprise one or more selected from the groupconsisting of wood chips, wood strands, and wood particles.
 5. Theprocess according to claim 1, wherein the gas flow is contacted with thewetted wood elements having a temperature of at least 120° C.
 6. Theprocess according to claim 5, wherein the temperature of the wetted woodelements decreases by at least 30° C. during said exposing of the wettedwood elements to the gas flow.
 7. The process according to claim 1,wherein the gas flow is contacted with a stream of the wetted woodelements containing 10 to 500 g water per 1 kg dry wood elements,wherein the wood elements comprise one or more selected from the groupconsisting of wood chips, wood strands, and wood particles, wherein thegas flow is contacted with the wetted wood elements having a temperatureof at least 120° C., and wherein the temperature of the wetted woodelements decreases by at least 30° C. during said exposing of the wettedwood elements to the gas flow.
 8. The process according to claim 1,wherein said gas flow comprises less than 5% oxygen by volume.
 9. Theprocess according to claim 1, further comprising: conveying the wettedwood elements through an evaporation section during said exposing thegas flow.
 10. The process according to claim 9, wherein said coolingfurther comprises: spraying water over the acetylated wood elements tohomogenously wet the acetylated wood elements; and after the spraying,providing the gas flow over the homogeneously wetted wood elements insaid evaporation section, the gas flow comprising less than 5% oxygen byvolume.
 11. The process according to claim 10, wherein said gas flow isprovided in cross-flow with the wetted wood elements.
 12. The processaccording to claim 11, wherein said evaporation section comprises ascrew section having a screw, and wherein movement of the screw providesfor transport and homogenization of the wetted wood elements.
 13. Theprocess according to claim 10, wherein the acetylated wood elements havea temperature of 150° C. or, an acetyl content of more than 20%, and aresidual acid content of less than 0.5 wt. %, and wherein the water isliquid water, wherein the spraying of the water comprises spraying theacetylated wood element with liquid water in an amount of 10-500 g waterper 1 kg dry wood elements to homogeneously wet the acetylated woodelements.
 14. The process according to claim 1, further comprisingstoring and/or handling the cooled wood elements.
 15. The processaccording to any of claim 9, further comprising: recirculating gas ofsaid gas flow through a recirculation loop coupled to said evaporationsection, wherein, in said recirculation loop, water vapour is condensed,and condensate is withdrawn from the recirculation loop.
 16. The processaccording to claim 15, wherein at least 3 wt. % water is condensed inthe recirculation loop based on dry weight of the wood elements.
 17. Acooling system, comprising: a water spray chamber, comprising: a waterspray chamber inlet for wood elements; a liquid distributor for sprayingwater; and a water spray chamber outlet for the wood elements, anevaporation section downstream of said water spray chamber, theevaporation section comprising: an evaporation section inlet and anevaporation section outlet for the wood elements; a conveyor forcontinuously transporting the wood elements from said evaporationsection inlet to said evaporation section outlet; an inlet conduithaving at least one opening for introducing gas into said evaporationsection; and an outlet conduit having at least one opening forwithdrawing the gas from said evaporation section, wherein said conduitsare coupled to a gas recirculation loop, and wherein said openings ofsaid conduits are different from said evaporation section inlet and saidevaporation section outlet for the wood elements.
 18. The cooling systemaccording to claim 17, wherein said recirculation loop comprises acooler, a condensation vessel, a recirculation loop outlet forcondensate, and a recirculation fan.
 19. A wood acetylation plant,comprising: a wood acetylation section having an outlet; and a coolingsystem downstream of the wood acetylation section, the cooling systemcomprising: a water spray chamber, comprising: a water spray chamberinlet for wood elements, the outlet of the wood acetylation sectionbeing coupled to the water spray chamber inlet for the wood elements; aliquid distributor for spraying water; and a water spray chamber outletfor the wood elements an evaporation section downstream of said waterspray chamber, the evaporation section comprising: an evaporationsection inlet and an evaporation section outlet for the wood elements; aconveyor for continuously transporting the wood elements from saidevaporation section inlet to said evaporation section outlet; an inletconduit having at least one opening for introducing gas into saidevaporation section; and an outlet conduit having at least one openingfor withdrawing the gas from said evaporation section, wherein saidconduits are coupled to a gas recirculation loop, and wherein saidopenings of said conduits are different from said evaporation sectioninlet and said evaporation section outlet for the wood elements.